Transparent substrate having a stack of thin metallic reflection layers

ABSTRACT

A glass pane is provided with a multilayer system having metallic reflection and a high thermal resistance. The multilayer system comprises a dielectric base layer, a metal layer having a high reflection, made of chromium or a metal alloy containing at least 45 wt % chromium, and a nitride top layer. The dielectric base layer is composed of at least one oxide partial layer made of SnO 2 , ZnO and/or TiO 2 , close to the surface of the glass, and a nitride partial layer, close to the metal layer.

[0001] The invention relates to a transparent substrate having amultilayer coating comprising thin layers for metallic reflection andfor high thermal resistance, these substrates are in particular suitablefor being heat-treated (bending, toughening, annealing), without theappearance and the optical properties of the layers being significantlyimpaired. The multilayer coating comprises a dielectric base layer, ametal layer having a high reflection, made of chromium or a metal alloycontaining at least 45 wt % chromium and a dielectric nitride coveringlayer. However, the invention also relates to “glazing” in which thesubstrate carrying the multilayer coating is transparent but not made ofglass, especially a rigid substrate of the polyethylene terephthalate(PET) type, therefore based on organic polymers.

[0002] Substrates coated in this way, having a high metallic reflectionand a relatively low light transmission, are very decorative. Inarchitecture, they are often used for dressing walls, or curtainwalling, (glazing panels or facing tiles), as mirror elements, assemitransparent mirrors or as decorative glass sheets. They may also beprovided with an additional decorative printing and/or may also be used,where appropriate, in curved or deformed form. If they are used asmonolithic glass panes, the surface layer is exposed, without anyprotection, to the atmosphere in such a way that it must have aparticularly high capacity to resist atmospheric effects. For safetyreasons and/or for the purpose of increasing their flexural strength andtheir impact strength, the coated glass substrates intended for theseapplications are often thermally prestressed, that is to say heated to atemperature above 500° C., 550° C. or 600° C., and then very rapidlycooled. The reflecting multilayer coating comprising thin layers mustalso withstand this thermal stressing without being damaged, theproperties, in particular the esthetic, optical, thermal and energyproperties, of the coated glass glazing having not to be disturbedthereby.

[0003] A glass pane with a high thermal resistance is known, fromdocument EP 0 962 429 A1, which has a multilayer coating of the kindmentioned at the beginning and which meets these requirements. In thecase of this known multilayer coating, the dielectric base layer iscomposed of SiO₂, Al₂O₃, SiON, Si₃N₄ or AlN, or else a mixture of atleast two of these materials. All these materials can be applied to theglass pane only with a relatively low sputtering rate and/or only with arelatively high technological effort being supplied. Because industrialcoating plants operate, for economic reasons, at run speeds as high aspossible, the base layer can be applied, in the case of standard plants,only with a relatively small thickness. However, a larger thickness ofthe base layer may be desirable as an interference layer to modify theappearance in light reflection (color and intensity of the color) of theglass pane, both in terms of light reflection on the glass side and interms of light transmission.

[0004] A multilayer coating capable of being curved and/or stressedbeforehand, having a chromium layer as reflection layer, is also knownfrom the document EP 0 436 045 A1. In this case, however, the coveringlayer is composed of an alloy of Al with Ti and/or Zr. Thanks to a toplayer of this kind, the high reflectivity of the chromium layer on thelayer side is lost and the reflection on the glass side reaches amaximum value of 50%. The base layer in this case is composed of TiO₂,Ta₂O₅, ZrO₂ or SiO₂. Admittedly, the base layer must, in this case, beable to be deposited with a thickness such that it acts as acolor-modifying interference layer, the increase in the layer thicknessbeing, however, limited for these materials for technological andeconomic reasons.

[0005] Document EP 0 536 607 B1 describes coated glass panes with ametallic appearance, the transparent coatings of which are also suitablefor a heat treatment. In this case however, the layer having metallicproperties is composed of a metal compound, namely a metal boride, ametal carbide, a metal nitride or a metal oxinitride. These metalcompounds do not possess the same gloss in light reflection as purelymetal layers. Deposited on this layer is a protective metal layer, forexample made of chromium, which is oxidized during the heat treatment.The multilayer coatings described in this publication are, firstly,coatings with a high transparency and the use of metallic reflection onboth sides of the glass panes for decorative reasons plays no role inthe case of these multilayer coatings.

[0006] It is desirable to ensure that the impression of color of thelight reflected by interferential action of the base layer is adjustablein a targeted manner, without it being necessary in this case to performtechnological feats or to reduce the run speed of the glass pane in acoating plant working continuously. A multilayer coating of this kindshould have a high corrosion (mechanical, chemical) resistance and ahigh hardness and should maintain, after a heat-treatment process of thebending, toughening or annealing type, its properties, in particular ahigh light reflection, to be essentially neutral in color on the layerside and to have a transmission lying within the range between 2 and15%.

[0007] The objective of the invention is to provide an improvedmultilayer coating of the kind mentioned at the beginning, having a highmetallic reflection on both sides of the glass panes thus coated.

[0008] This objective is achieved according to the invention thanks tothe features of claim 1. The secondary claims provide advantageousimprovements to the subject matter of the present invention.

[0009] The dielectric base layer is consequently composed of at leastone oxide partial layer, close to the surface of the glass, having areflective index ≧2.0, and of a nitride partial layer which may or maynot be contiguous with the metal layer.

[0010] A base layer simply based on an oxide easily sputterable bysputtering has not proven itself because it turns out, that in thiscase, the chromium metal layer undergoes a color modification during theheat treatment and is even destroyed. This risk is completely eliminatedby the formation of multiple layers according to the invention of thebase layer, during which a thin partial layer made of a metal nitride isplaced immediately beneath the metal layer. Thanks to the second partiallayer being placed so as to be contiguous with the surface of the glassand based on a metal oxide easily sputterable by sputtering, having areflective index of at least 2.0, the thermal resistance of themultilayer coating is not disturbed. The appearance in light reflection(color and color intensity) of the reflected light may be modified in asimple manner within wide ranges. The run speed of the glass panesthrough the coating plant must, however, not be substantially reduced.Oxide partial layers of this kind may be deposited without any problemwith a thickness ranging up to 90 nm, so that there is considerable playwith regard to the appearance in light reflection that may be obtained.

[0011] As an improvement of the invention, the nitride partial layer ofthe dielectric base layer is composed of Si₃N₄ and/or AlN (whichoptionally may contain minority elements (Al, boron etc.) which aredopants of the targets) and has a layer of at least 10 nm. The oxidepartial layer or layers of the dielectric base layer is (are) preferablybased on SnO₂ and/or ZnO and/or TiO₂ and/or Nb₂O₅ and/or ZnO₂ and has(have) a thickness ranging from 30 to 90 nm.

[0012] A particularly high gloss of the coated glass panes is obtainedby using pure chromium for the metal layer having a high reflection.Thanks to pure chromium layers, it is possible to achieve degrees ofreflection in the visible range of the spectrum ranging up to 60%.However, CrAl alloys containing 75-80 wt % Cr, CrSi alloys containing45-85 wt % Cr and CrAlSi alloys containing 70-80 wt % Cr are alsoappropriate as chromium-containing metal alloys.

[0013] By way of example, the following operating methods can be used toobtain the metal oxide layers and the nitride layers:

[0014] if the deposition is reactive, by using a suboxidized metaltarget (in the case of metal layers), by using an Si metal target(possibly doped with Al or boron) or using a subnitrided target (in thecase of nitride layers);

[0015] if the deposition is nonreactive, ceramic targets may be used (inthe case of metal layers) or by using a nitrided target (in the case ofnitride layers).

[0016] In an advantageous improvement of the invention, a 1 to 3 nmthick protective layer made of a metal or a metal alloy, such as Zr, Ti,TiCr, ZrCr or TiNi, is placed on the nitride covering layer of themultilayer coating. This protective metal layer is converted, during theheat treatment, so as to form a corresponding oxide layer.

[0017] The multilayer coatings according to the invention can be appliedto the glass panes as integral layers adhering to one another over theirentire area. Particularly esthetic effects can be obtained when thereflecting multilayer coating is applied to the surface of the glass asa discontinuous layer in the form of a decoration or pattern, or ifpatterns or decorations made, for example, of a color to be baked areprinted on that side of the glass pane which has been coated. The colorto be baked may be annealed after the heat-treatment process.

[0018] Further advantages and details of the invention will becomeapparent from the description which follows of three examples of theinvention, to be compared with two comparative examples according to theprior art.

[0019] As tests intended to evaluate the corrosion resistance of themultilayer coating, the following were carried out: the “salt fog” testin accordance with the ISO 9227 standard, the “SO₂” test in accordancewith the DIN 50018 standard, the “Taber abrasion” test in accordancewith the DIN EN 1096-1;-2 standard, the “sweat” test in accordance withthe DIN 50017 standard and the “Cass” test in accordance with the ISO9227 standard. Measurements of the transmission T_(vis) and thereflection R_(vis), in the visible range of the spectrum, and the colorcoordinates a* and b*, for the purpose of determining the color inreflection, were carried out in accordance with the DIN 5033 standard.

COMPARATIVE EXAMPLE 1

[0020] Panes of flute glass, 6 mm in thickness, having the surfacedimensions of 6×3.21 m², were coated in an industrial sputtering plantaccording to the process of magnetically enhanced sputtering, with themultilayer coating comprising: glass/10 nm of SiO₂/35 nm of Cr/6 nm ofSi₃N₄. Specimens taken over the width of the glass panes gave thefollowing reflection values R_(vis) and transmission values T_(vis):R_(vis): coating side 57.0% glass side 48.7% T_(vis):  2.5% Color inreflection (laboratory system): coating side a* = −0.36 b* = 1.43  glass side a* = −0.53 b* = 0.32. 

[0021] The colors in reflection on the glass side and on the multilayercoating side are almost identical, the multilayer coating side having,however, a slight yellowish coloration, as always evinced by thepositive b* value of 1.43.

[0022] The tests relating to the corrosion resistance and to thehardness of the multilayer coating gave the following results: Salt fogtest: passed SO₂ test: passed Taber abrasion test: passed Sweat test:passed Cass test: passed.

COMPARATIVE EXAMPLE 2

[0023] Attempts were made to coat the glass panes with the samemultilayer coating in the same sputtering plant as that used in the caseof Comparative Example 1, with however a base layer having a thicknesssuch that the color in reflection on the glass side was blue. For thispurpose, it was necessary to apply the SiO₂ base layer with a thicknessof 100 nm. Despite the use of special Si cathodes, it was possible tomanufacture the 100 nm thick base layer only with a substantiallyreduced run speed of the glass pane.

[0024] Measurement of the optical properties gave the following values:R_(vis): coating side 57.3% glass side 28.3% T_(vis):  3.0% Color inreflection (laboratory system): coating side a* = −0.5 b* = +1.5 glassside a* = −1.1 b* = −8.3.

[0025] Compared with Comparative Example 1, there was a substantialdecrease in the reflection on the glass side. Furthermore, theproductivity was considerably reduced during coating. However, all thecorrosion and hardness tests were passed without any problem, as in thecase of Comparative Example 1.

ILLUSTRATIVE EXAMPLE 1 OF THE INVENTION

[0026] The following multilayer coating according to the invention:glass/58 nm of SnO₂/17 nm of Si₃N₄/35 nm of Cr/6 nm of Si₃N₄/2 nm of Zrwas produced on the same large sputtering plant. Various specimens wereextracted from the coated glass pane over the width of the pane. Thespecimens were subjected to a heat treatment. Measurement of the opticalproperties on the prestressed specimens gave the following results:R_(vis): coating side 57.4% glass side 30.0% T_(vis):  3.5% Color inreflection (laboratory system): coating side a* = 0.13    b* = 4.03   glass side a* = −0.01  b* = −0.96.

[0027] All the corrosion and hardness tests were passed without anyproblem. The color in reflection on the glass side was neutral and themultilayer coating side had a particularly high reflection with a brightyellow and pleasing tint.

EXAMPLE 2 OF THE INVENTION

[0028] The following multilayer coating according to the invention:glass/42 nm of SnO₂/8 nm of ZnO/17 nm of Si₃N₄/17 nm of Cr/18.5 nm ofSi₃N₄/2 nm of Zr was produced in the same cycle of tests, underidentical test conditions moreover.

[0029] The following optical properties were determined: R_(vis):coating side 38.4% glass side 19.0% T_(vis): 14.0% Color in reflection(laboratory system): T_(vis): coating side a* = −0.39  b* = 16.8   glass pane side a* = −0.86  b* = −11.6.

[0030] All the corrosion and hardness tests were passed without anyproblem. On the glass side, the color in reflection was an intense blue,whereas on the multilayer coating side the color was yellow. A paneelement of this kind can be used as a semitransparent mirror, forexample as an integral glass door, one side appearing blue in reflectionand the other side appearing yellow.

EXAMPLE 3 OF THE INVENTION

[0031] The following multilayer coating according to the invention:glass/56 nm of SnO₂/34 nm of Si₃N₄/35 nm of Cr/6 nm of Si₃N₄/2 nm of Zrwas produced in the large sputtering plant, under the identical testconditions moreover, and the properties of the layers were determined.

[0032] Determination of the optical properties gave the followingvalues: R_(vis): coating side 55.5% glass side 39.5% T_(vis):  2.5%Color in reflection (laboratory system): coating side a* = 0.13   b* =3.98   glass side a* = −6.53 b* = −2.0. 

[0033] All the corrosion and hardness tests were passed without anyproblem. The glass element appeared in reflection, on the glass side,green-blue and, on the multilayer coating side, it had a high reflectionwith a yellowish tint. A glass element of this kind can be used, forexample, with the glass side facing the outside, as a curtain wallingelement, the multilayer coating side having in this case to fulfill nodecorative function.

1. A transparent substrate having a multilayer coating comprising thinlayers for metallic reflection and for high thermal resistance, thissubstrate being in particular suitable for being heat-treated (bending,toughening, annealing), which comprises a dielectric base layer, a metallayer having a high reflection, made of chromium or a metal alloycontaining at least 45 wt % chromium and a dielectric nitride coveringlayer, characterized in that the dielectric base layer is composed of atleast one oxide partial layer, which may or may not be contiguous withthe transparent substrate, having a reflective index ≧2.0, and of anitride partial layer contiguous with the metal layer.
 2. The substrateas claimed in claim 1, characterized in that the oxide partial layer orlayers of the dielectric base layer is (are) based on SnO₂ and/or ZnOand/or TiO₂.
 3. The substrate as claimed in claim 1 or 2, characterizedin that the oxide partial layer or layers of the dielectric base layerhas (have) an overall thickness of 30 to 90 nm.
 4. The substrate asclaimed in any one of claims 1 to 3, characterized in that the nitridepartial layer of the dielectric base layer is based on Si₃N₄ and/or AlN,and optionally on minority elements.
 5. The substrate as claimed in anyone of claims 1 to 4, characterized in that the nitride partial layer ofthe dielectric base layer has a thickness of at least 10 nm.
 6. Thesubstrate as claimed in any one of claims 1 to 5, characterized in thatthe chromium-containing alloy is a CrAl alloy containing 75-80 wt % Cr,a CrSi alloy containing 45-85 wt % Cr or a CrAlSi alloy containing 70-80wt % Cr.
 7. The substrate as claimed in any one of claims 1 to 6,characterized in that the chromium or chromium alloy metal layer has athickness of 15 to 50 nm.
 8. The substrate as claimed in any one ofclaims 1 to 7, characterized in that a 1 to 3 nm thick protective layermade of a metal or a metal alloy, such as Zr, Ti, TiCr, ZrCr or TiNi, isplaced on the nitride covering layer, which protective layer isconverted, after the heat treatment of the substrate thus coated, into acorresponding oxide layer.
 9. The substrate as claimed in any one ofclaims 1 to 8, characterized in that the nitride covering layer is basedon Si₃N₄ and/or AlN and has a thickness of at least 6 nm.
 10. Thesubstrate as claimed in any one of claims 1 to 9, characterized in thatthe multilayer coating is applied to the substrate as a continuouscoating.
 11. The substrate as claimed in any one of claims 1 to 9,characterized in that the multilayer coating is applied to the substrateas a discontinuous layer in the form of a decoration or pattern.
 12. Thesubstrate as claimed in any one of claims 1 to 11, characterized in thatthe substrate thus coated is heat-treated in a mechanical or chemicaltreatment.